Breathlessness after knee arthroplasty in stage 4 CKD: is the CTPA cancelled?
A 68-year-old woman presents to the emergency department at 21:00 with acute breathlessness and pleuritic chest pain, four days after an uncomplicated total knee arthroplasty. Heart rate 106/min, blood pressure 128/78 mmHg, respiratory rate 24/min, SpO2 93% on room air, BMI 38. The chest radiograph is reported as normal. She has stage 4 chronic kidney disease with an eGFR of 27 mL/min/1.73 m², no previous contrast reaction and no prior contrast studies. Her Wells score is 6 ('PE likely'). The referring team requests a CT pulmonary angiogram and asks whether the study should be cancelled because of her renal function. A SPECT V/Q service is available on site, but only during working hours.
1. Should the CTPA be cancelled because of the eGFR of 27 mL/min/1.73 m²? Justify your answer and state how you would modify the study.
Read the worked answer
No - the study should go ahead.
Reasoning:
- Reduced eGFR is a risk marker for contrast-associated acute kidney injury, not a contraindication. The absolute risk in CKD 4 is modest and the great majority of patients do not develop clinically significant injury.
- The alternative to imaging here is not 'no risk' - untreated PE in a patient with a high pre-test probability carries a mortality in the region of 20–30%, and she has additional risk factors (recent arthroplasty, obesity, hypoxaemia). The net clinical benefit clearly favours diagnosis.
- Trials of prophylaxis (bicarbonate, NAC, IV saline) have largely failed to show benefit, so there is no need to delay the scan for a hydration ritual.
- A V/Q SPECT would be a legitimate alternative, but at 21:00 it is not available and a delay of 12 hours is its own hazard.
Practical modifications:
- Use a low-osmolar agent at the lowest volume that gives diagnostic opacification (typically 50–70 mL at 300–350 mgI/mL, with a saline chaser and bolus tracking).
- Do not plan any further contrast study in the next 48–72 hours; document the decision and the reasoning in the notes.
- Ensure adequate hydration, avoid NSAIDs and nephrotoxins, and check renal function at 48–72 hours.
2. Assume the chest radiograph instead showed a peripheral wedge-shaped opacity. How does that change your choice of test, and what does the D-dimer add?
Read the worked answer
It pushes the choice further towards CTPA.
- V/Q scintigraphy relies on a normal or near-normal chest radiograph for interpretability. With a wedge-shaped opacity (possible infarction), mismatched defects become much harder to call and the proportion of non-diagnostic studies rises substantially.
- In the PIOPED II population, an abnormal radiograph was one of the strongest predictors of a non-diagnostic V/Q scan; CTPA retains high accuracy regardless.
- The wedge opacity also raises the possibility of an alternative or additional diagnosis (pneumonia, infarction with cavitation), which CTPA assesses directly.
Regarding D-dimer: with a Wells score of 6 the test is not usable to rule out PE. In high pre-test probability, a negative D-dimer has a poor negative predictive value and should not be used to withhold imaging. Its only role would be if the clinical probability had been reassessed as low.
3. CTPA confirms a segmental PE with an RV:LV ratio of 1.2 and a small pericardial effusion. Blood pressure is 128/78 mmHg. Troponin is mildly elevated. Discuss anticoagulation and whether thrombolysis is indicated, taking her renal function into account.
Read the worked answer
This is intermediate-risk (submassive) PE, not high-risk - systemic thrombolysis is not indicated.
Reasoning:
- She is haemodynamically stable. The PEITHO trial showed that routine fibrinolysis in intermediate-risk PE prevents haemodynamic collapse but increases major bleeding and stroke, with no mortality benefit. So: anticoagulate, admit for monitoring, and keep thrombolysis in reserve as rescue therapy if she deteriorates (falling BP, rising lactate, worsening hypoxaemia, or progressive RV failure).
- The RV:LV ratio of 1.2 plus a raised troponin defines intermediate-high risk rather than intermediate-low; that means a monitored bed and a low threshold for escalation, not a syringe of alteplase.
Anticoagulation with eGFR 27:
- Apixaban is licensed down to creatinine clearance 15 mL/min and is a reasonable choice, used with caution in CKD 4. Dabigatran and edoxaban should be avoided at this level of renal function.
- Low molecular weight heparin (with anti-Xa monitoring if very obese - BMI 38 is relevant) or unfractionated heparin are safe alternatives.
- Avoid NSAIDs, review all nephrotoxic drugs, and re-check renal function, since the PE itself and any deterioration will affect clearance. If she does deteriorate, her renal function does not preclude rescue thrombolysis - the bleeding risk, not the kidney, is the deciding factor.